/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * An iterator that supports a one-element lookahead while iterating.
 *
 * <p>
 * See the Guava User Guide article on
 * <a href= "https://github.com/google/guava/wiki/CollectionHelpersExplained#peekingiterator">
 * {@code PeekingIterator}</a>.
 *
 * @author Mick Killianey
 * @since 2.0
 */
@GwtCompatible
public interface PeekingIterator<E> extends Iterator<E> {
    /**
     * Returns the next element in the iteration, without advancing the iteration.
     *
     * <p>
     * Calls to {@code peek()} should not change the state of the iteration, except that it
     * <i>may</i> prevent removal of the most recent element via {@link #remove()}.
     *
     * @throws NoSuchElementException if the iteration has no more elements according to
     *         {@link #hasNext()}
     */
    E peek();

    /**
     * {@inheritDoc}
     *
     * <p>
     * The objects returned by consecutive calls to {@link #peek()} then {@link #next()} are
     * guaranteed to be equal to each other.
     */
    @CanIgnoreReturnValue
    @Override
    E next();

    /**
     * {@inheritDoc}
     *
     * <p>
     * Implementations may or may not support removal when a call to {@link #peek()} has occurred
     * since the most recent call to {@link #next()}.
     *
     * @throws IllegalStateException if there has been a call to {@link #peek()} since the most
     *         recent call to {@link #next()} and this implementation does not support this sequence
     *         of calls (optional)
     */
    @Override
    void remove();
}
